In looking at the construction block diagram of a optical coherence
tomograph, the source is defined as a "low coherence interferometry".  The
light is sent to the retina and this light is compared with reflected light.
And the comparisons are made by comparing phase differences.

 Question, is not (or would not) a high coherence source be better?  I am
not sure I understand the basic optical principle.  Any pointers, links,
hints appreciated.

Don W.